The invention relates to a vehicle air conditioning system with a compressor according to the precharacterizing clause of claim 1.
Air conditioning systems of this sort have previously been operated with coolants such as R12 or R134a. The operation of such a system requires the compressor to be capable of producing a pressure difference of ca. 10 to 20 bar between suction pressure and counterpressure. This can be achieved with compressor pistons comprising no or at most one piston ring.
Organic halogen coolants have recently been publicly criticised because when released into the atmosphere, they can have an ozone-decomposing action and aggravate the greenhouse effect. Therefore considerable efforts have been made to develop air conditioners that operate with coolants that pose no risk to ozone, i.e. are neutral with respect to the greenhouse effect. Here the problem arises that substances potentially suitable as coolants, inasmuch as they meet these requirements, demand considerably higher pressure differences. Such differences are not readily achievable with the conventional constructions of air-conditioner compressors. When an attempt is made to employ such coolants in a compressor with a conventionally constructed piston, without piston ring or with a single piston ring, the efficacy of such a compressor is seriously impaired by leakage losses owing to flow of the coolant along the skirt of the piston, and there is a risk that these will make the system entirely nonfunctional. Therefore in the case of air conditioning systems employing coolants that must be greatly compressed, cylinders have been used in which the pistons bear as many as four rings. A compressor for a vehicle that comprises one cylinder is disclosed, for example, in the U.S. Pat. No. 4,594,055. In order to improve the seal between piston and cylinder, the piston comprises on its skirt two grooves that run in the circumferential direction, with piston rings positioned in each. In comparison to a piston with only one piston ring, this piston is more difficult to tilt within the cylinder, as a result of which wear and tear on the inner cylinder wall is reduced. The seal between piston and cylinder is also improved. However, this increases the effort and expense of constructing the compressor. Furthermore, the friction of the piston against the inner walls of the cylinder is considerably increased, so that operation of the compressor requires extra work that contributes nothing to the cooling action. On the contrary, the frictional heat generated between piston and cylinder is largely absorbed by the coolant, which is absolutely undesirable.
These disadvantages are not present in the air conditioning unit with the features given in Claim 1. Additional advantageous embodiments of the invention will be evident from the dependent claims.
By means of the invention an automobile air conditioning system is created that, by limiting leakage losses in the compressor, enables operation with coolants that must be greatly compressed, while avoiding an appreciable increase in friction as compared to air conditioners designed for smaller pressure differences.
Such an air conditioner is particularly well suited for operation with CO2 as coolant in the so-called transcritical process, or with other coolants that require large differences between suction pressure and counterpressure, ranging from over 20 bar up to 100 bar.
In a first air conditioning system in accordance with the invention, the piston is provided with a labyrinth seal at its skirt surface. Such a labyrinth seal, which preferably comprises a plurality of grooves oriented in the circumferential direction of the skirt surface, is also suitable for reducing leakage losses particularly in the case of high pressures. The reason is that it is just when the pressure of the coolant contained in the cylinder is highest, a time when particularly large leakage losses are to be expected, that the viscosity of the coolant is no longer negligible because of the severe compression, and as a result the turbulence of the coolant escaping from the cylinder between its inner wall and the piston skirt, with the grooves of the labyrinth seal, presents an effective resistance to flow.
In a second embodiment of the air conditioning system in accordance with the invention, in which the piston is conventionally constructed insofar as it comprises at least one slot within which a piston ring is mounted, it is provided that there is additionally disposed in the slot a ring made of a flexible sealing material. The function of this ring is to constrict or completely block the path of the coolant, which in the case of a piston with piston ring runs through the slot substantially along its outward-facing surfaces and along the inner circumferential surface of the piston ring.
For this purpose the ring is disposed substantially between the piston ring and a side wall of the slot at which the pressure is relatively low, so that the piston ring is pressed by the coolant against the flexible ring, as a result of which a sealing action of the latter is achieved. The flexible ring itself advantageously does not extend outward beyond the surface of the piston skirt, so that it does not come into contact with the inner wall of the cylinder; therefore it is not subject to frictional wear and tear and also does not increase the force needed to overcome friction in order to drive the piston.
It is further preferred that the piston ring be held within the slot with axial play. This can enable the coolant to penetrate from the high-pressure side into a space between the inner surface of the piston ring and the bottom of the slot, and thus to press the piston ring uniformly, over its entire circumference, against the inner wall of the cylinder. It can be sufficient in this regard for this play to be generated only in a high-pressure phase of the working cycle of the compressor, by axial compression of the ring that is made of flexible sealing material.
A space can also be provided between the bottom of the slot and the inner surface of the ring made of flexible sealing material, so that the latter, just like the piston ring, is pressed radially outward by coolant that penetrates into the space and thus reinforces the corresponding extension movement of the piston ring.
Additional characteristics and advantaces will be apparent from the following description of exemplary embodiments.